Shielded high-frequency cable



April 1950 E. A. J. MOL 2,503,834

SHIELDED HIGH-FREQUENCY CABLE Filed lay 9, 1946 INVEN TQR EVERT ARY JAN MOL ATTORNEY Patented Apr. 11, 1950 UNITED STATES PATENT OFFICE y memo ents,to

Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application May 9, 1946, Serial No. 668,417 In the Netherlands February 2, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires February 2, 1962 4 Claims. (Cl. 174-28) For the construction of so-called shielded highfrequency cables the attempts are directed to as low as possible a capacity between the internal conductor or conductors and the external shielding means or external conductor. When filling the space between internal conductor or conductors and shielding means or external conductor with some insulating material, for instance a thermoplastic synthetic material on the basis of polyvinylchloride or acetyi-cellulose the comparatively high dielectric constant of the said insulating materials involves an excessive capacity. unless the distance between internal conductor o conductors and shielding means or external conductor or conductors is given an impracticable size. Moreover, such a cable involves very great dielectric losses.

A solution may be found in the well-known airspace insulation. In this case the space between internal conductor or conductors and shielding means or external conductor is only partly filled with some insulating material and the part of the said space occupied by air is as large as possible.

This can be achieved; for instance, by surround ing the internal conductor or conductors with a system of wires, bands or strips of paper or cellulose derivatives. Thus an insulation is obtained which has suitable electrical properties in a dry state. The dielectric constant of a paper-air space insulation amounts to about 1.6. However, a serious drawback of this construction is its high sensitiveness to moisture which requires very expensive measures to be taken in manufacture. The cable made by means of paper must not only-be protected against penetration of moisture by providing a surrounding thimble or lead but, moreover, all moisture must be expelled from the paper insulation before providing the thimble of lead.

In another well-known solution of the problem the internal conductor or conductors is surrounded with a much wider tube consisting of a more or less flexible material about which tube were provided the shielding means or the external conductor. For this flexible cable use is made, for instance, of thermo-plastic materials on the basis of polyvinylchloride and ethylcellulose. Cables thus made have fairl suitable working capacities, but the dielectric losses of such cables are considerable. Moreover, the range of temperature, within which these cables can be used without the risk of damaging Or deformation is comparatively small and certainly does not extend beyond the range comprised between C. and +80 2 whereby the use of antenna cables made by means of these materials is limited.

According to another well-known solution endowed with much better properties both in electrical and mechanical respect the above-mentioned insulating tube is replaced by a helix consisting of polystyrene. The dielectric losses as well as the dielectric constant of polystyrene are very low and practically not dependent on moisture. Moreover, the compression strength of a polystyrene helix is to be considered excellent and the helix which in the case of bending is mainly under strain of torsion, has suilicient flexibility. However, all these good properties are opposed by the drawback of the range of temperature, within which the use is possible without damaging or deformation, being comparatively small also in this case, where the bottom limit of the range of temperature is lower than in the above-mentioned case, but the upper limit does not much exceed C. and certainly not C. In fact, above this temperature polystyrene begins to soften and the cable, which is indebted for its mechanical strength to the presence of the polystyrene helix, may easily undergo a permanent deformation due to which, of course, the electrical properties and particularly the capacity will be aflected.

An insulating material, which exhibits extremely suitable electrical properties as a result of its chemical structure and is but slightly sensitive to moisture so that it is particularly suitable for replacing the insulating material already referred to above, is an insulating material built up on the basis of poiyisobutylene having a molecular weight of 100,000 to 200,000 which is very advantageous for this purpose. This material, as a hydrocarbon, has a low dielectric constant e and very dielectric losses; 6 is of the order of magnitude of 2.5 and the dielectric losses are practically constant over a. large temperature and frequency range. The loss angle amounts to 0.0002. Considering the mechanical properties of polyisobutylene, more particularly the very low compression strength of this hydrocarbon, it is not possible, however, to use the polyisobutylenes without providing the cable with special strengthening means, in view of the condition of the compression strength of the cable and, consequently, its resistance to deformation being as high as possible, in addition to as low as possible a capacity, as low as possible dielectric losses and as low as possible a sensitiveness to moisture.

According to the invention a high frequency 0., 65 cable having a low capacity and a great comby it at certain points,

pression strength, which conserves its mechanical and electrical properties over a very large temperature range, is obtained by providing that the insulation between the internal conductor or conductors and the shielding means of the external conductor consists of air and polyisobutylene, preferably having a molecular weight of 100,000 to 200,000, which for attaining the required compression strength is surrounded by a helically wound strengthening wire consisting of elastic material which conserves its elasticity in the temperature range.

The helically wound strengthening wire may consist of steel wire surrounded by the shielding means of the external conductor. When very stringent conditions are imposed on the losses and damping the elastic material should preferably not be a magnetic material, a helically wound hard copper wire or in some cases a helically wound bronze wire being chosen instead of the steel helix. It is of importance to provide this helically wound wire of elastic material under the actual screening means or external conductor, since therefor a braiding of thin soft copper wires will be used, or in some cases aluminum wire in order to obtain the required flexibility. When the shielding means are directly applied to the tube of polyisobutylene followed by surrounding it with the strengthening wire then, on bending the conductor, the shielding means or external conductor may rather easily be deformed, so that the insulating tube may become depressed or in serious cases even be perforated which will cause capacity variations.

In order that the invention may be more readily understood and carried into effect it will now be described with reference to the accompanying drawing in which like numerals refer to like parts and in which:

Fig. 1 is a plan view of the cable according to the invention; and

Fig. 2 is an end view of the cable according to the invention.

As an example of the invention reference is made to the drawing, wherein i denotes the internal copper conductor which may sometimes be provided with a layer of enamel. This internal conductor l is surrounded by a much wider tube of polyisobutylene 2 surrounded by a helically wound hard copper wire 3. The reference number 4 designates a surrounding braiding of soft copper wire which may be covered with a texture of paper and a textile braiding such as cotton. The assembly may still be impregnated with a water repellent and abrasion resistant impregnating agent such as asphalt. The internal conductor l is loose from the tube 2 and supported However, the internal conductor may be secured to additional supporting members such asdiscs of insulating material.

The cable according to the invention has excellent electrical properties. The dielectric constant of this air-space insulation lies below that of a paper-air space insulation, but the losses are very low. In addition, to these excellent electrical properties the cable according to the invention has excellent mechanical properties which affords the particular advantage that the range of temperature, within which the cable can be used without the risk of damaging or permanent deformation is very extensive and comprised between -50 C. and +180 C.

While I have thus described my invention with specific examples and applications other obvious modifications thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.

What I claim is:

l. A shielded high frequency cable comprising a conductor element, a tubular member of polyisobutylene substantially filled with air and having a molecular weight between approximately 100,000 and 200,000 surrounding said conductor, and a helically wound elastic strengthening wire surrounding said tubular member.

2. A shielded high frequency cable comprising a conductor element, a tubular member of polylsobutylene substantially filled with air and having a molecular weight between approximately 100,000 and 200,000 surrounding said conductor, and a helically wound hard copper wire surrounding said tubular member.

3. A shielded high frequency cable comprising a conductor element, a tubular member of polyisobutylene substantially filled with air and having a molecular weight between approximately 100,000 and 200,000 surrounding said conductor, and a helically wound bronze wire surrounding said tubular member.

4. A shielded high frequency cable comprising a conductor element, a tubular member of polyisobutylene substantially filled with air and having a molecular weight between approximately 100,000 and 200,000 surrounding said conductor, a helically wound hard cop r wire surrounding said tubular member, a soft copper braid surrounding said hard copper wire, and an outer water repellent covering over said cop r braid.

EVERT ARY JAN MOL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

